N2 - Dredge plume modelling is a key tool for quantification of potential impacts to inform the EIA process. There are, however, significant uncertainties associated with the modelling at the EIA stage when both dredging methodology and schedule are likely to be a guess at best as the dredging contractor would rarely have been appointed. Simulation of a few variations of an assumed full dredge period programme will generally not provide a good representation of the overall environmental risks associated with the programme. An alternative dredge plume modelling strategy that attempts to encapsulate uncertainties associated with preliminary dredging programmes by using a scenario-based modelling approach is presented. The approach establishes a set of representative and conservative scenarios for key factors controlling the spill and plume dispersion and simulates all combinations of e.g. dredge, climatic and spill scenarios. Impact zones are derived from the modelling of the individual scenarios through established impact criteria for key environmental receptors, and an “envelope” of impact zones is derived by combining all the individual scenarios. The impact envelopes for different receptor types readily identifies receptors at risk, while a wealth of information that can be extracted from the individual scenarios forms a solid platform for taking environmental objectives into account in optimisation of the final dredge programme. Requirements for the approach to be applied as well as advantages gained from the scenario approach are discussed, and the
typical application as part of an EIA outlined.

AB - Dredge plume modelling is a key tool for quantification of potential impacts to inform the EIA process. There are, however, significant uncertainties associated with the modelling at the EIA stage when both dredging methodology and schedule are likely to be a guess at best as the dredging contractor would rarely have been appointed. Simulation of a few variations of an assumed full dredge period programme will generally not provide a good representation of the overall environmental risks associated with the programme. An alternative dredge plume modelling strategy that attempts to encapsulate uncertainties associated with preliminary dredging programmes by using a scenario-based modelling approach is presented. The approach establishes a set of representative and conservative scenarios for key factors controlling the spill and plume dispersion and simulates all combinations of e.g. dredge, climatic and spill scenarios. Impact zones are derived from the modelling of the individual scenarios through established impact criteria for key environmental receptors, and an “envelope” of impact zones is derived by combining all the individual scenarios. The impact envelopes for different receptor types readily identifies receptors at risk, while a wealth of information that can be extracted from the individual scenarios forms a solid platform for taking environmental objectives into account in optimisation of the final dredge programme. Requirements for the approach to be applied as well as advantages gained from the scenario approach are discussed, and the
typical application as part of an EIA outlined.